Personal health monitors provide users with the ability to monitor their overall health and fitness by enabling the user to monitor heart rate or other physiological information during exercise, athletic training, rest, daily life activities, physical therapy, etc. Such devices are becoming increasingly popular as they become smaller and more portable.
A heart rate monitor represents one example of a personal health monitor. A common type of heart rate monitor uses a chest strap monitor that includes surface electrodes to detect muscle action potentials from the heart. Because such surface electrodes provide a relatively noise free signal, the information produced by monitors that use surface electrodes is highly accurate. However, most users find chest strap monitors uncomfortable and inconvenient.
Another type of heart rate monitor uses PPG sensors disposed in an ear bud. The ear provides an ideal location for a monitor because it is a relatively immobile platform that does not obstruct a person's movement or vision. PPG sensors proximate the ear have, e.g., access to the inner ear canal and tympanic membrane (for measuring core body temperature), muscle tissue (for monitoring muscle tension), the pinna and earlobe (for monitoring blood gas levels), the region behind the ear (for measuring skin temperature and galvanic skin response), and the internal carotid artery (for measuring cardiopulmonary functioning). The ear is also at or near the point of the body's exposure to environmental breathable toxins of interest (volatile organic compounds, pollution, etc.), noise pollution experienced by the ear, lighting conditions for the eye, etc. Further, as the ear canal is naturally designed for transmitting acoustical energy, the ear provides a good location for monitoring internal sounds, such as the heartbeat, breathing rate, mouth motion, etc.
PPG sensors measure the relative blood flow using an infrared or other light source that projects light that is ultimately transmitted through or reflected off tissue, and is subsequently detected by a photodetector and quantified. For example, higher blood flow rates result in more light being scattered by the blood, which ultimately increases the intensity of the light that reaches the photodetector. By processing the signal output by the photodetector, a monitor using PPG sensors may measure the blood volume pulse (the phasic change in blood volume with each heartbeat), the heart rate, heart rate variability, and other physiological information. PPG sensors are generally small and may be packaged such that they do not encounter the comfort and/or convenience issues associated with other conventional health monitors. However, PPG sensors are also more sensitive to motion artifact noise than are many other types of sensors, and thus are more prone to accuracy problems.